Crystal Structure, Steady-State, and Pre-Steady-State Kinetics of Acinetobacter baumannii ATP Phosphoribosyltransferase.

The first step of histidine biosynthesis in Acinetobacter baumannii , the condensation of ATP and 5-phospho-α-d-ribosyl-1-pyrophosphate to produce N 1 -(5-phospho-β-d-ribosyl)-ATP (PRATP) and pyrophosphate, is catalyzed by the hetero-octameric enzyme ATP phosphoribosyltransferase, a promising target for antibiotic design. The catalytic subunit, HisG S , is allosterically activated upon binding of the regulatory subunit, HisZ, to form the hetero-octameric holoenzyme (ATPPRT), leading to a large increase in k cat . Here, we present the crystal structure of ATPPRT, along with kinetic investigations of the rate-limiting steps governing catalysis in the nonactivated (HisG S ) and activated (ATPPRT) forms of the enzyme. A pH-rate profile showed that maximum catalysis is achieved above pH 8.0. Surprisingly, at 25 °C, k cat is higher when ADP replaces ATP as substrate for ATPPRT but not for HisG S . The HisG S -catalyzed reaction is limited by the chemical step, as suggested by the enhancement of k cat when Mg 2+ was replaced by Mn 2+ , and by the lack of a pre-steady-state burst of product formation. Conversely, the ATPPRT-catalyzed reaction rate is determined by PRATP diffusion from the active site, as gleaned from a substantial solvent viscosity effect. A burst of product formation could be inferred from pre-steady-state kinetics, but the first turnover was too fast to be directly observed. Lowering the temperature to 5 °C allowed observation of the PRATP formation burst by ATPPRT. At this temperature, the single-turnover rate constant was significantly higher than k cat , providing additional evidence for a step after chemistry limiting catalysis by ATPPRT. This demonstrates allosteric activation by HisZ accelerates the chemical step.